Parts for industrial furnaces and similar installations need to be made from alloys of moderate cost yet require one or more of the properties of resistance to hot gas corrosion, carburization, thermal fatigue or thermal shock failure, along with hot strength and the capability of being melted and poured in air. These parts have mostly been produced from alloy compositions standardized by the Alloy Castings Institute (ACI) Division of the Steel Founders Society of America and designated by an H followed by one other letter to differentiate between types. By far the most widely employed of these ACI alloys have been the HH type with nominal contents of about 25% Cr and 12% Ni and the HK type, nominally containing about 25% Cr and 20% Ni. When rapid or repeated thermal cycling was to be encountered in service, the HT type of about 35% Ni and 16% Cr was often employed. Most of the other ACI-type alloys have been very much less employed.
All of the ACI-type alloys have only moderate hot strength, but type HP, of about 35% Ni and 25% Cr content, combines good hot gas corrosion resistance with greater hot strength than any of the other ACI-types. Still, there have been ever increasing needs for alloys of even greater hot strength, and, in some instances, for further improvement in hot gas corrosion resistance or carburization resistance.
Efforts to answer these needs have resulted in the development of various modifications of standard ACI-type alloys. The most widely modified ACI alloy has been type HP, which has been altered by additions of about 1% to 5% W, and sometimes increases in nickel content up to about 48%, alternatively with about 15% Co, yielding a nickel plus cobalt content of about 48%. Some modifications employ increased chromium content to about 28%, while other modifications also employ one or more of each of columbium, molybdenum or titanium in amounts of about 2% or less. Unfortunately, alloys containing nickel plus cobalt in amounts up to a total of 48% along with about 5% W suffer from a significant increase in cost. There has, therefore, been considerable effort directed toward development of ACI-type alloys which are strengthened and improved by relatively small additions of component elements from the group W, Mo, Cb, Zr, Ti, Al, V, Ca, N, B, Mg, Ta, Th, Be, Cu, and Ce or some combination of rare earth elements in place of Ce. Such additions, when employed in quantities of the order of less than about 1% each by weight have come to be called microalloy additions.
In addition to the efforts to improve the hot strength of ACI-type alloys many alloys having excellent hot strength have been developed for applications in aircraft gas turbine engines, with rotary blade and stator vane materials in the turbine stage presenting the most demanding requirements. Such alloys are almost entirely produced in vacuum or inert gas atmospheres and contain large total quantities of relatively scarce and expensive elements. However, the maximum hot strength of these alloys is obtained with resultant sacrifice in ductility, machinability, weldability and hot gas corrosion resistance. In general, these alloys are far too costly and not well suited for industrial furnace parts.
Other alloys containing from about 33% to about 60% Cr by weight have been developed either to extend operating temperatures up to 2200.degree. F. to 2400.degree. F. or to provide resistance to particularly corrosive gases containing compounds of such elements as vanadium, sodium and sulfur. In order to achieve maximum hot corrosion resistance, however, these alloys have sacrificed other properties and suffer from one or more of the very undesirable characteristics of low hot strength, greatly increased costs, excessive brittleness and poor fabricability, weldability, formability and foundry properties. They represent the opposite end of the spectrum of properties from the gas turbine alloys.
As is known to those in the art, there is no perfect alloy for all heat resistant applications. Every alloy represents a compromise of properties and of constituent elements, but the most desirable alloys for industrial furnace parts and similar applications require both high hot strength and high hot gas corrosion resistance along with moderate cost and long service life.
By 1940, many workers in the heat resistant alloy field had reported improvements in the hot gas corrosion resistance of heat resistant alloys brought about by additions of the order of a half percent or less of such elements as calcium, magnesium, zirconium, thorium and cerium or other rare earth elements, which are often supplied commercially in a mixture called mischmetal. These additions improved the protective surface coatings of the alloys which naturally form in the presence of oxygen, and refined the grains of the alloys.
Post, et al, U.S. Pat. No. 2,553,330, discloses improvements in the hot workability of virtually all corrosion and heat resistant alloys by the addition of about 8 to 12 pounds per ton of molten metal of cerium, lanthanum or other rare earth elements. Since these elements float on the surface of the molten bath and readily oxidize in air, Post's additions were said to result in recoveries in the final solid metal of only about 0.14% to about 0.32% by weight of rare earth elements. Post also teaches that recovery of substantially larger amounts of cerium and other rare earth metals results in deterioration in fabricability to levels below those of the original alloys without any rare earth metal addition.
Scharfstein, U.S. Pat. No. 3,168,397, claims benefits in corrosion resistant alloys by the addition of about 0.1% to about 0.3% of rare earth elements to the final metal.
Heyer, et al, U.S. Pat. No. 4,077,801, discloses substantial improvements in hot strength of ACI-type alloys by the addition of about 0.5% W and about 0.3% Ti, along with the possible inclusion of columbium (niobium). These alloys are marketed by the Manoir Electralloys Corporation, formerly by the Abex Corporation, under the trade name Thermax.
Japanese patent J6 0059-051A describes what is essentially the ACI HP alloy base plus 0.5 to 3% W, 0.2 to 0.8% Mo, 0.3 to 1.5% Cb, 0.04 to 0.5% Ti, 0.02 to 0.5% Al and small amounts of B and N. An examplary alloy contains nominally 1% Cb, 1% W, 0.4% Mo, 0.15% T, 0.15% Al, 0.08% N and 0.002% B. This alloy is then subjected to "coating" under controlled conditions to diffuse large amounts of aluminum into the "skin" of the alloy. Typically, when ready to go into service, this alloy will contain from about 0.3% to about 0.8% Al within the first millimeter of surface depth and about 0.15% to about 0.4% Al in the layer from 1 mm to 2mm depth. The resultant alloy is said to have excellent resistance to heat and carburization. While the alloy in this patent is said to contain large quantities of molybdenum, tungsten, columbium and titanium without pronounced tendency to form other matrix phases that shorten service life, such amounts of these elements would be too much for the HF, HH, HI, HK and HL types of ACI alloys, all of which are only borderline stable, with their standard nickel and chromium contents, before any other ferrite-forming elements are added. Also this Japanese patent specifies additions of aluminum, another very strong ferritizing element which would further tend to destabilize these alloys having borderline stability.
Present day nickel-base superalloys do not contain any appreciable quantities of carbon and derive their hot strength by formation of precipitates from the solid solution matrix of nickel-aluminum-titanium compounds, referred to as gamma-prime phase. These alloys may contain up to 8% Al and up to 5% Ti. Since both of these elements are readily oxidized at molten alloy temperatures in air, all such alloys are produced in vacuum or some inert gas atmosphere.
Aluminum and all of the carbide forming elements, chromium, molybdenum, tungsten, cobalt, titanium, zirconium, hafnium, tantalum and vanadium oppose, or destabilize in various degrees, the desired austenitic, or face-centered-cubic, crystal matrix structure of these alloys. Thus, when these elements are present in sufficiently large amounts in aggregate they cause formation of such non-austenitic phases, either in production or in service, as alpha, delta, sigma, laves, mu or others, all of which lead to early loss of hot strength and failure in service.
Nickel, cobalt, carbon, nitrogen and manganese all tend to promote or maintain the desired austenitic matrix of these alloys. Therefore, increasing amounts of the so-called ferrite-formers mentioned above may to some extent be offset by increasing amounts of the austenite-formers. But, there are numerous limitations. For example, nickel is moderately expensive, and many of the ACI-type alloys contain as little as 8% Ni and, usually, large amounts of iron. Cobalt may partially substitute for nickel in this role, but it is considerably scarcer than nickel and generally much more expensive.
Manganese and nitrogen have been employed, often as partial nickel substitutes in corrosion resistant alloys, which operate at or near room temperatures. A high manganese content is generally detrimental to hot strength of heat resistant alloys, and manganese is limited to about 2% maximum as a deoxidizing component in ordinary steelmaking practice. While nitrogen has beneficial effects upon corrosion resistance in certain media, it is less beneficial than carbon in developing hot strength in heat resistant alloys. Since both nitrogen and carbon in large amounts reduce ductility and weldability, carbon is primarily chosen for strengthening corrosion resistant super alloys.
Chromium is required in ACI-type and similar alloys to provide resistance to oxidation in air or in other typical service atmospheres. Nickel is of some benefit in this regard for most of the hot gases typically encountered, so that a somewhat lower chromium content may be tolerated in alloys of very high nickel content. For example, type HF alloy begins to scale badly above about 1650.degree. F, while type HT, of higher nickel but slightly lower chromium content than type HF, resists scaling to about 1950.degree. F. While nickel is much more expensive than chromium, alloys of high nickel content are nevertheless employed because of their increased hot strength. It is not cost effective to attempt to reduce chromium by increasing nickel in the desire for only better hot gas corrosion resistance. It is more cost effective to employ high Cr to Ni ratios if hot gas corrosion resistance is mainly required at lower hot strengths. If hot strength is also an important factor in a given application, somewhat higher Ni to Cr ratios may be employed to attain the same level of hot gas corrosion resistance.
It is therefore seen that in ACI-type alloys cobalt, manganese, carbon and nitrogen all have certain practical limits, and so therefore do the possible combinations and amounts of the ferrite-forming elements from which increased hot strength is derived.
Some alloys produced early in the development of super alloys for the gas turbine and turbo jet industries are listed in Table I.
TABLE I ______________________________________ WEIGHT % OF ELEMENTS DESIGNATION Ni Cr Co Mo W Cb Ti ______________________________________ S-495 20 15 -- 4 4 4 -- S-497 20 15 20 4 4 4 -- S-590 20 20 20 4 4 4 15 S-816 20 20 45 4 4 4 -- N-153 15 16 13 3 2 1 -- N-155 20 20 20 3 2 1 0.25 U.S. Pat. No. 2,416,515 9 19 -- 1.4 1.4 0.4 0.25 U.S. Pat. No. 3,127,265 35 28 15 -- 5 -- -- (SUPERTHERM) ______________________________________
In the "S" series of alloys, the contents of molybdenum, tungsten and columbium represent substantial additions of ferrite-forming, carbide-forming elements. At 1200.degree. F. for periods up to about 1000 hours, the hot strength of the "S" alloys increases with increasing amounts of nickel and cobalt. However, the first three alloys of Table I were quite unstable and were reduced to about the same much lower strength levels for 1000 hour periods at 1350.degree. F. Only the S-816 alloy, of much higher total nickel and cobalt content, was found to be sufficiently stable metallurgically over longer periods of time to continue in use.
The N-153 and N-155 alloys were of lower molybdenum, tungsten and columbium content, but the N-153 alloy still contained too much of these three elements, when coupled with the lower amounts of nickel and cobalt, to be stable over long periods of time even at the reduced chromuim level. The N-155 alloy continued in use for decades for moderately low temperature service of about 1350.degree. F. or less, because it is metallurgically quite well balanced and stable.
Evans, U.S. Pat. No. 2,416,515, is also shown in Table I. That patent discloses an alloy having even lower amounts of molybdenum, tungsten and columbium, along with a small amount of titanium and a very nominal nickel content of 9%. But this low-nickel alloy is still unstable metallurgically even with its much reduced content of ferrite formers. At 1600.degree. F. over periods beyond 1000 hours or at 1500.degree. F. over periods beyond 16 months the '515 alloy has even lower hot strength than the ACI plain HF 30 alloy, which contains none of the four elements, molybdenum, tungsten, columbium and titanium, but is otherwise the same base alloy. In view of the fact that ACI-type alloys are expected to last for years at temperatures generally above 1500.degree.-1600.degree. F. the alloys of Table I, including Evans '515, do not teach quantities of the elements molybdenum, tungsten, columbium and titanium, that are useful in enhancing hot strengths and service lives of ACI-type alloys.
Present day super alloys have been formulated to contain various combinations not only of the above elements, molybdenum, tungsten, columbium and titanium, but also of as much as 0.2% B, 2.5% V, 2.25% Zr, 9% Ta, 2% Re, 0.5% Hf and small amounts of berylium, yttrium or lanthanum. The other elements found in the super alloys are present in various combinations of 0 to 68% Co, 0 to 78% Ni, 3 to 28% Cr, 0 to 17% Mo, 0 to 20% W, 0 to 6% Cb, 0 to 8% Al and 0 to 5% Ti.
More recently Manoir Electroalloys Corporation has produced ACI-type HK and HP alloys, which evidence further improved hot strength as a result of additions of about 0.5% W, 0.25% Cb, 0.10% Ti and some addition of cerium or other rare earth element. These alloys are marketed under the trade names TMA 4700 and TMA 6300 for the improved HK and HP alloys, respectively.
Nevertheless, in spite of these various efforts to provide alloys of increased hot strength, there remains an enormous demand for further improvements in the properties of these alloys. Especially attractive is the attainment of those properties by microalloying due to the attractive low cost for gains in properties in addition to hot strength. Thus, it is particularly desirable to achieve even further improvements in hot strength as well as improved hot ductility, weldability and resistance to thermal fatigue and thermal shock, without sacrifices in machinability or foundry properties.